1. Field of the Invention
This invention relates to a method and apparatus for compressing digital audio signals of a first format, such as those of a compact disc format, and transforming the compressed signals into second digital audio signals of a different format.
2. Description of the Related Art
The optical disc may be increased in recording capacity by two to three orders of magnitude larger than the magnetic disc so that it permits high-speed accessing as compared to the tape-shaped recording medium. Besides, the optical disc renders it possible to record and/or reproduce information signals without contacting the optical pickup unit as the recording and/or reproducing apparatus with the disc and hence is superior in durability.
As such optical disc, a so-called compact disc, referred to hereinafter as CD, is currently in extensive use.
To provide a portable and, above all, a pocket size headphone stereo or similar recording/reproducing apparatus using an optical disc, a CD with a diameter of 12 cm and a CD with a diameter of 8 cm have been proposed. However, with a disc diameter of 12 cm, the recording/reproducing apparatus become too bulky in size to be readily portable. For further reducing the size of the recording/reproducing apparatus, it may be contemplated to employ a disc which is 8 cm or less in diameter. However, in attempting to construct a portable or pocket size recording/reproducing apparatus for an optical disc which is 8 cm or less in diameter, the following problems arise.
In the case of a standard CD format optical disc, on which are recorded stereophonic digital PCM audio signals with a sampling frequency of 44.1 kHz and quantized with 16 bits, and where these signals are exclusively reproduced by the user, the playback time (recording time) is 20 to 22 minutes at most, such that a symphony, for example, cannot be recorded on one disc side. A playback time of 74 minutes or longer, which is approximately equal to that of a 12 cm CD, is desired. Besides, with this CD-DA format, recording by the user is not feasible. In addition, a contact-less type optical pick-up device is vulnerable to mechanical vibrations and subject to detracking or defocusing. Thus, when the apparatus is to be portable, some positive means must be provided to prevent any adverse effects of detracking or defocusing on signal reproduction.
On the other hand, with a CD-MO format (a format employing a recordable magneto-optical disc) as an extension format of the above-mentioned standard CD format (CD-DA format), the recording/reproducing time of an 8 cm disc is only 20 to 22 minutes, as in the CD-DA format. The optical pick-up device is similarly subject to detracking or defocusing due to mechanical vibrations, such that again some positive means must be taken to prevent any adverse effects thereof on signal recording/reproduction.
With the CD-I (CD-Interactive) format, signals obtained by 4-fold compression of standard CD-DA format signals are reproduced. Thus, if all of the recorded data are stereophonic audio compressed data, playback time increases 4-fold, or reproduction of 4-channel data, becomes feasible, so that reproduction for 70 minutes or longer becomes possible with an optical disc which is 8 cm in diameter. However, when continuous signals are to be recorded with the CD-I format, several track jumps over a considerable distance spanning the inner and outer disc peripheries need to be carried out. Since the track jump cannot be achieved instantaneously, playback data are interrupted for the time being, that is, the playback sound is interrupted momentarily. Besides, if continuous audio signals are to be recorded, it becomes necessary to re-write previously recorded sector signals, because the signals are interleaved during recording. Thus it becomes extremely difficult to record continuous compressed signals.
In view of the foregoing, the present Assignee has proposed in JP Patent KOKAI Publication No.4-105271 (1992), a disc recording apparatus and a disc play-back apparatus for overcoming the above-depicted problems. That is, an arrangement is disclosed in which 16-bit digital audio signals are compressed to one-fourth of one-fifth to enable recording and/or reproduction of music signals of 72 to 76 minutes on or from a magneto-optical disc having a disc diameter of 64 mm.
Meanwhile, with an audio compression technique for producing compressed digital audio signals recorded and reproduced on or from the magneto-optical disc which is 64 mm in diameter, that is the adaptive transform acoustic coding (ATRAC), analog signals are translated into digital signals of a bit rate of 1.4 Mbps which is the same as that for the DC format. Subsequently, the digital signals are processed with frequency analysis of transforming the time domain signals into approximately 1,000 frequency components by the Fourier transform, with data of approximately 20 ms at the maximum as one block. During the frequency analysis, extraction of the frequency components on the frequency domain is performed, beginning from those components which are most critical to the aural sense, using the so-called minimum audibility characteristics or masking effects, based on the knowledge of the aural psychology, until the amount of information becomes equal to 300 kbps.
Meanwhile, the disc recording and/or playback apparatus for recording/reproducing compressed digital audio signals on or from the magneto-optical disc which is 64 mm in diameter have already been manufactured and put on sale. The discs in an unrecorded state for recording by the user, or the pre-recorded disc having the music, for example, recorded thereon, are available on the market.
For producing the pre-recorded discs, such as music discs, it may be contemplated to produce newly a compressed format master tape for compressed digital audio signals to record the digital audio signals from such master tape to the disc, by way of mastering. However, it is more desirable from the viewpoint of saving the labor of re-recording of the music or reducing the costs to utilize the pre-recorded master tapes for production of CDs, which are available in large quantities.
Consequently, it becomes necessary to produce a new master tape in which the CD format digital audio data from the preexisting master tapes for CD production is transformed in accordance with a compression format for compressed digital audio signals.
It is noted that, when producing a master tape, the audio level to be recorded has to be set to an optimum or desirable audio level. The sound level value is of particular importance for professional systems. Consequently, when producing a master tape of the compression format for the compressed digital audio signals, it is necessary to calculate and display the audio level values and to adjust the displayed audio level.
However, the signal level of the original pre-compression signals cannot be monitored correctly by simply transforming the CD format digital audio signals into data of a compression format for compressed digital audio signals, recording the transformed data on a master tape, reproducing the compressed digital audio signals with decompression or expansion, calculating and displaying the produced signal level. That is, since the original pre-compression signals are changed in signal level by compression, the signal level after compression and subsequent expansion cannot be said to correctly represent the level of the original signal. In other words, if the digital audio signals are compressed, the ratio of the signal level before compression to that after compression is not necessarily equal to 1.
Bedsides, if the compressed digital audio signals are decompressed or expanded and the level value is computed based on such signals, it becomes necessary to carry out the level value computation and the signal processing for display at an increased speed compatible with the compression ratio.